wu4c9 by adel khamis

8/14/2019 wu4c9 by Adel Khamis

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Unit Four -131- Chapter nine

The Electric Current & Ohms Law

Electric Current:

....

..

Electric current intensity (I):

t

QI=

Definition: .

Rule: ..

Dimensional formula:

Units: . Or ..

N.B.:

In our study we will deal with the conventional direction of current,

which consider the electric charge is (+ve / -ve) charge.

The direction of the electric current

through the external circuit is from

the . pole to the

.. pole of battery.

The direction of the electric current inside the battery is from the

pole to the .. pole.

Potential difference between two points (V):

ech

workdifferencePotential

arg=

Definition: .

Work Sheet 2008/2009


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Rule: .

D.F.: ..

Units: . or or ..

The Electro motive force (e.m.f):

Q

WE =

Definition: .

Or: .

Unit: or .

If the electromotive force is 8 volts, then the potential difference is

(greater than equals to less than) 8 volts.

Resistance (R):

Definition: .

Unit: .. symbol of the unit: .

Parameters affecting the electric resistance:

a) Length of the conductor:

The resistance is (directly / inversely) proportional to the length of the conductor.

.........R

........L

........LRR

2

1=

Where the rest of factors are constant.

b) Cross sectional area of the conductor:

The resistance is (directly / inversely) proportional to the cross sectional area of the

conductor.



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........R

....

.......

2

1

A

A

R

R=

Where the rest of factors are constant.

c) Resistance depends on kind of the matter:

Therefore:

The relation between resistance and length: ..........R

The relation between resistance and area: ..........R

From the previous two equations: ...........R

Therefore: ...........=R

.......

..............=R

Where is the resistivity; and it depends on the kind of matter.

Resistivety (specific resistance) ():Rule: ..

Definition:


Unit: ...

Conductivity ():Definition:

Rule: ..


Unit: ..



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Question:

What are the factors that affect each of resistance and resistivity?

..

..

..

Examples:

1) What will happen to the resistance of a resistor when its length increases

to its double and its cross sectional area decrease to its half.

[Increases 4 times]

2) What are the resistivity and the electrical conductivity of a copper wire of

length 25 meters, diameter 0.09 cm and resistance 0.67 .

.

[1.7 x 10-8m, 5.87 x 107Simon/m]

3) When a current of 2 A flows through an unknown resistor, the potential

difference across it will be 10 volts. Calculate the resistivity of the

material of such resistor if its length is 2 meters and its cross sectional

area is 0.1 cm2.



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[2.5 x 10-5m]

4) Calculate the resistance of a copper wire of length 2 meters and cross

sectional area 0.05 cm2 if the resistivity of cooper is 1.7 x 10-3m.

.

[680 ]

Electric circuit:

It consists of:

...

...

..

...

...

Ohms law:

..

..


V

I


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..

..

..

..

IV

IconstV =

RIV =

Ohm:

Definition: .

Unit: ..

Example:

In Ohms experiment the following table obtained:

V in volts 1 A 2 2.5 3.5 5

I in mA 200 300 400 B 700 1000

Draw (V-I graph) and from the graph find:

The value of (a) and (b).

The value of the fixed resistance.



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a = . , b = ..

Slope = .. , R = .

Connecting the resistors:

There are two ways for connecting resistors:

Resistors in series:

Explain an experiment to determine the equivalent resistance for

group of resistors connected in series.

..

..

..

..

The electric current intensity (divided / does not divided).

The potential difference between the groups is (equal / not equal) to

summation of potential differences between each resistor.

I = .

V = (1)

From Ohms law

V = . (2)

From (1) and (2)



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I R = .

I R = I (..)

R = R1 + R2 + R3

If the resistors are equals:

R = N r

Where:

r is ..

N is .

R is .

Resistors in Parallel:

Explain an experiment to determine the equivalent resistance for

group of resistors connected in parallel.

..

..

..

..

The potential difference (divided / does not divided).

The electric current intensity of the group is (equal / not equal) to

summation of electric current intensity of each resistor.



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Vt = .

It = .. (1)

From Ohms law

I = . (2)

From (1) and (2)

.

.......

1

.......

1

.......

11++=

R

If the resistors are equal

.........

1 N

Rt=

........

.......=tR

In case of two resistors:

21

111

RRR+=

21

..............1

RRR

+=

.............

............

+

=R

N.B.:

In case of connection in series the total resistance is greater than the

greatest resistor. (G.R)

..

In case of connection in parallel the total resistance is less than the

smallest resistor. (G.R)



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..



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Ohms law for a closed circuit:

)( rRIVB +=

................. +=B

V

......+=VVB

Where:

E: is ...

R: is

r: is .

Glossary:

W = VIT

P = VI

Where:

I: is . and its unit is

Q: is . and its unit is

t: is . and its unit is

V: is . and its unit is

W: is . and its unit is

R: is . and its unit is

P: is . and its unit is


t

QI =

Q

WV =

I

VR=


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1) Three resistors of 25 , 70 and 85 are connected in series to a 45

volts battery on negligible internal resistance. Calculate, the current

flowing in each resistor and the terminal P.d of each.

.

[0.25 A, 6.25 V, 17.5 V, 21.25 V]

2) If the resistors in the previous problem are connected in parallel to the

same battery, calculate:

a. The current flowing in each resistor.

b. The total resistance.

c. The current through the circuit.

[1.8 A, 0.64 A, 0.529 A , 15.14 , 2.97 A]



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3) Two resistors A and B are connected in parallel, the combination is

connected in series to a resistor C and an 18 volt battery of negligible

internal resistance. If the resistance of A, B and C are 3, 6, 7

respectively. Calculate the total resistance, the current flowing through

the circuit and through each of A and B.

[9 , 2 A, 4/3 A, 2/3 A]

4) A cell of E.M.F 2 volts and internal resistance of 0.1 is connected to a

circuit of external resistance 3.9 Calculate the current flowing through

the circuit.

[0.5A]

5) Calculate the total resistance

of the circuit shown in the


10

20

30

20 40

12 V6 V


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figure and the total current flowing through the circuit if the internal

resistance of each cell is 2 .

The previous figure can be drown as:

.

[20 , 0.75 A]


10

20

30

20 40

12 V6 V

wu4c9 by adel khamis

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